It has been demonstrated that the multi-layer ceramic technologies (MLC) can be used very effectively with RF communication devices. One problem in using this technology is only moderate Q can be obtained for stripline resonators fabricated using current MLC processes. By way of example, FIG. 1 and FIG. 2 show a conventional stripline resonator 10 consisting of dielectric substrates 12 which is metallized on a first side 11 and a second side 13 and includes an embedded center strip conductor 14.
The center conductor may be shaped either in a straight fashion or meandered, zig-zagged or spiraled in a line in the longitudinal direction. If a fixed substrate height and center conductor width are used, the Q of the stripline resonator increases with a corresponding increase in center conductor thickness. This is due to the perimeter of the center conductor cross-section which is enlarged so more conductor area is available to pass RF currents. This initial gain in Q, with increased center conductor thickness, will eventually be canceled due to the reduced dielectric volume, which is the energy storage media for RF signal propagation.
The thickness of the stripline center conductor 14 fabricated using current MLC processes, and/or stripline in general, is usually very thin, i.e. less than 1 mil. One method used to fabricate thick center conductors is the so called "trough-line" approach. This method is shown in FIG. 3 which depicts, a trough 21 carved on a ceramic tape 23. The trough 21 is then filled with a metal paste (not shown). This produces a thick trough line which has been successfully fabricated in the laboratory with encouraging results. One problem associated with the trough line technique is it's difficulty to implement in a mass-production environment. This is due to the shape of the trough 21 extending in the longitudinal direction where it is limited to a few simple shapes to maintain the integrity of the carved ceramic tape.
With the migration of MLC technologies to high tier RF products, many components such as voltage controlled oscillators (VCO) and filters were limited by these low Q factors. It has been determined that the lower Q of the MLC stripline resonators is due to many factors. These include:
1) A low dielectric Q associated with low-fired glass ceramic materials;
2) Impurities added to silver metal paste used for greater adhesion and shrinkage match to ceramic tapes; and
3) Screen printed metal traces which are relatively thin and formed sharp edges after lamination and pressing so metal loss increases due to current bunching at sharp edges and corners sometimes called the proximity effect.
Therefore, to obtain better quality MLC stripline resonator Q, a low-loss, low-fired glass ceramic material, high purity silver metal paste is needed. Further, a means and method is needed to increase metal trace thickness and to alleviate the proximity effect in the stripline structure.
Prior art techniques have relied on thick trough lines in the stripline. These have been successfully fabricated in the laboratory with encouraging results. The present invention provides a simple and cost effective way to fabricate an effective thick MLC stripline resonators by printing two vertically aligned conductor traces which are electrically connected by vias. This results in a 20-30% improvement in resonator Q. Also, the invention does not require new processing techniques and additional fabrication steps and is in compliance with current MLC processing techniques used in the industry. It allows an improvement in MLC stripline resonator Q using MLC technologies allowing production of high-tier RF components.